Well, the only way you're getting decapitated or otherwise similarly maimed (losing arms etc) is:
1) Specific magic weapons - Swords of Sharpness, Vorpral Blades, etc
2) A monster/trap/effect has a specific rule for that happening.
3) Such maiming is, or is pretty much, the only logical outcome of some action a player has their character take.
4) You get killed by massive damage.
5) You get killed by a crit.
In #s 4 & 5 the decapitation/maiming is the description of your death, not what caused it rules-wise. Though yes, story-wise, death occurred because of the decapitation.
And it might be important later on that you were decapitated etc....
You'd think that #2 would be the most common method. But no. In all my years playing these games I've seen ALOT more instances of #3 (mostly maimings).
#1 is the least likely way for these things to occur in my games.
What I do NOT use in my games are random crit charts